Disband only those units that are actually using food upkeep
from starving city. Units that have free upkeep, usually provided
by government type, are not disbanded.

Partch by Guillaume Melquiond <silene>

See patch #3902


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@22865 a0f10bec-cc02-0410-94fc-a9cfff90b4cd